Abstract: Elastic instability of rigid-jointed, unbraced, single-story single-bay, orthogonal space frames subjected to static loads at the column tops is studied. The columns are assumed to be fixed at the base. The influence on the frame behavior of the relative magnitude of the load-multiplication factors for the four columns and the orientation of the principal axes of the column cross-sections is investigated. The instability loads are also compared with those obtained approximately using the effective-length approach. Interesting conclusions are drawn based on the results of 12 numerical examples.